The present invention relates to a field effect transistor as described in the preamble of claim 1, a control method realized by such a field effect transistor as described in the preamble of claim 2 and a frequency mixer means including such a field effect transistor as described in the preamble of claim 3.
Such a field effect transistor is already known in the art, e.g. from the book xe2x80x9cElectronic Principles, Second Editionxe2x80x9d published, 1979, by Tata McGraw-Hill Publishing Company Limited and Printed at Pearl Offset Press, Kirti Nagar, Industrial Area, New Delhi-110015. An active electronic device such as described more particular in chapter 13 xe2x80x9cField-effect transistorsxe2x80x9d of this book includes a control terminal called a gate, a first main terminal called a source and a second main terminal called a drain. This first paragraph of this chapter teaches how a gate voltage which is applied to the gate of the field effect transistor controls a flow of carriers which is flowing through a channel from the source to the drain. This means that a field effect transistor is a voltage-controlled device i.e. input voltage alone controls the output current.
It has to be remarked that the use of the terms source and drain is the same for n-channel or p-channel field effect transistors which means that for n-channel field effect transistors the carriers are conduction-band electrons and that for the p-channel field effect transistors the carriers are holes in the inversion region. Hence, conventional current flows from the drain to the source in n-channel field effect transistors and from the source to the drain in p-channel field effect transistors.
Although, that such a field effect transistor has a lot of advantages, however, as described on page 322, second paragraph, a main drawback of such a field effect transistor is the small control over the output current. This means that a field effect transistor is less sensitive to changes in input voltage than e.g. a bipolar transistor.
An object of the present invention is to provide a field effect transistor of the above known type but which provides an improved controllability and an additional degree of freedom over the flow of carriers from the source to the drain.
According to the invention, this object is achieved by the field effect transistor as described in claim 1 which is realizing the control method as described in claim 2 and which is included in the frequency mixer means described in claim 3.
Due to the fact that the field effect transistor further includes a third main terminal, called hereafter a double drain, positioned and adapted in order to enable a high resistance control current means, which is coupled to the double drain, deviation of part of the flow of carriers from the source to the double drain is enabled.
Indeed, by coupling a high input resistance control current means like e.g. a current source to the double drain an extra control is provided to the drain current of the field effect transistor. The field effect transistor becomes in this way a device with mixed control inputs : one voltage control input at the gate of the field effect transistor and one current control input at the double drain of the field effect transistor. Both control inputs are controlling the drain current i.e. the output current of the field effect transistor.
It has to be explained that the normal working of the field effect transistor has to be respected in the event when no control input is required via the double drain terminal. In this way it is expected to have a high impedance at the double drain terminal which explains the use of a high resistance control current means like e.g. a source current. On the other hand, in the event when the control current means is active, it is important to enable a deviation of part of the carriers, even when this part constitutes all the present carriers, when such is required. The high resistance control current means at the double drain terminal has to be dimensioned in such a way that both requirements are possible to be fulfilled. This will be discussed in a further paragraph.
It also has to be explained that the expression xe2x80x9cpositioned and adaptedxe2x80x9d, used in the claims means that the double drain must be positioned as such that indeed a deviation of the carriers is possible. This will become more clear in the following paragraphs.
First, it should be clear that a distinction has to be made between e.g. the well known parallel drain transistor or the multiple finger transistor and the field effect transistor of this application. Indeed, the electrical field under the gate of a parallel drain transistor, formed by the flow of carriers from the source to one of the extra drains, has a direction which is substantially in parallel with the direction from the electrical field formed by the flow of carriers from source to first drain following the normal working of a field effect transistor. Such an extra field has the object to accelerate the charge in the channel whereby more carriers are flowing from the source to the drains. The electrical field of the present field effect transistor formed by the flow of carriers from source to second drain has a direction which is at least under an angle different from zero compared with the electrical field formed by the flow of carriers from source to first drain according to the normal working of the field effect transistor. This extra electrical field has the object to deviate the flow of carriers to the additional drain.
Yet, it should be clear that also a distinction has to be made between a double drain transistor such as e.g. used in the International Patent Application with publication number WO 9608041 A1 960314 and the double drain transistor of the present application. Indeed, the flow of carriers of the above mentioned patent application is switched from one drain to another drain based on the presence of a magnetic field. The object of this application is to measure time and space distributions of a magnetic field in a digital way.
Finally, it has to be explained that the result of a suited position of the double drain i.e. a deviation of the carriers towards the double drain, is one which can be directly and positively verified by tests which are known to the person skilled in the art. Indeed, by measuring the current of the first drain in the event when a plurality of different parts of the carriers are deviated under the control of a high input resistance control current means which is coupled to the double drain, a suited position of the double drain is simply confirmed.
Furthermore, such a suited positioning of the double drain does not require undue experimentation. Indeed, a first possible suited positioning of the double drain of the present application is e.g. in the same plane with the source and first drain but such that the direction gatexe2x80x94toxe2x80x94double drain is substantially perpendicular to the direction of the electrical field formed by the flow of carriers from source to first drain. In this case, the double drain is positioned closer to the source as the first drain which enables the current control means to deviate all the carriers, which are flowing from source to first drain, towards this double drain. A second possible suited positioning of the double drain is e.g. again in the same plane with the source and the first drain by such that a triangle is formed by the source, first drain and double drain round the channel.
It has to be remarked that an alternative solution to have an improved controllability over the drain current is the known dual gate field effect transistor. Such a dual gate field effect transistor controls the drain-source current through more than one gate electrode. Although that such a dual gate transistor is attractive to many circuit applications like e.g. mixer circuits and memory circuits, such a dual gate transistor is still a voltage control input device. An application of a dual gate field effect transistor is e.g. in the frequency mixer circuit as described in the above mentioned book xe2x80x9cElectronic Principles, Second Editionxe2x80x9d published, 1979, by Tata McGraw-Hill Publishing Company Limited and Printed at Pearl Offset Press, Kirti Nagar, Industrial Area, New Delhi-110015, and more particular in paragraph 23-8 Spurious signals, on pages 673 to 676. In this frequency mixer the incoming signal i.e. a radio signal is applied to one of the gates and a local oscillator i.e. a beat-frequency-oscillator drives the other gate in order to provide at the output of the mixer a signal with an audio frequency.
A possible application of the field effect transistor of the present invention is a frequency mixer means. This is described in claim 3. In such a frequency mixer means a first frequency is applied to the control terminal i.e. the gate and a second frequency is applied to the third main terminal i.e. the double drain. This application is in fact an example from what is mentioned in one of the above paragraphs not only part of the carriers, but all the carriers, which are flowing from the source to the first drain, are deviated to the double drain at predetermined time moments.
Furthermore it has to be remarked that although in the claims the presence of one extra drain is described, the present invention is not restricted to only one extra drain. Indeed, by providing more than one extra drain, each one being coupled to a high input resistance control current means, the deviation of part of the carriers is controlled through more than one extra drain electrode which provides an improved controllability and an additional degree of freedom over the flow of carriers from the source to the first drain.
It should be noticed that the term xe2x80x9cincludingxe2x80x9d, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x9ca device including means A and Bxe2x80x9d should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noted that the term xe2x80x9ccoupledxe2x80x9d, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x9ca device A coupled to a device Bxe2x80x9d should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and in input of B which may be a path including other devices or means.